linux-kernel - Re: [PATCH 0/8] perf: add ability to sample physical data addresses

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Message-ID: <20130625105123.GA13649@gmail.com>
Date:	Tue, 25 Jun 2013 12:51:23 +0200
From:	Ingo Molnar <mingo@...nel.org>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Stephane Eranian <eranian@...gle.com>,
	LKML <linux-kernel@...r.kernel.org>,
	"mingo@...e.hu" <mingo@...e.hu>,
	"ak@...ux.intel.com" <ak@...ux.intel.com>,
	Arnaldo Carvalho de Melo <acme@...hat.com>,
	Jiri Olsa <jolsa@...hat.com>,
	Namhyung Kim <namhyung.kim@....com>
Subject: Re: [PATCH 0/8] perf: add ability to sample physical data addresses


* Peter Zijlstra <peterz@...radead.org> wrote:

> On Tue, Jun 25, 2013 at 11:59:09AM +0200, Stephane Eranian wrote:
> > One guarantee we have is that the physical does correspond to the virtual
> > address at the time of the interrupt.
> 
> That might not be much of a guarantee depending on the circumstances.
> 
> > But yeah, if physical pages are swapped during the run, then things become
> > a lot more complicated. I am not trying to address this.
> > 
> > Can page move for shared memory segments?
> 
> Yep..
> 
> > > Such would completely shatter physical page relations.
> > >
> > > If the shared memory thing is really the issue, doesn't perf already
> > > have the process memory layout (/proc/$PID/maps and aux stream mmap
> > > updates) with which it can compute map relative offsets and compare
> > > thusly?
> > 
> > Not sure I understand this.
> > suppose the same shared memory segment is mapped at two different
> > addresses by shmat(). First, I don't know if those show up in /proc/maps.
> 
> They should; IIRC maps is a full vma list.. /me prods about in
> fs/proc/task_mmu.c.. yes it prints all vmas.

A syscall (ioctl?) to dump all current vmas into the mmap update stream 
(to form a starting point) might be handy - that would remove the 
fragility and overhead of parsing /proc/ details.

Thanks,

	Ingo
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